1. Field of the Invention
This invention relates to a process for producing a maraging steel cylinder for uranium enriching by a centrifugal separator and to cylinders produced thereby. More particularly, this invention relates to a process which uses drawing and ironing steps in producing the cylinders.
2. Description of the Prior Art
Known processes for producing enriched uranium, which are used for atomic power plants, are the gas diffusion process and the centrifugal separation process. Centrifugal separation has recently become popular because of the rapid development and progress in that process. Uranium enriching using a centrifugal separator utilizes the small difference in mass of U.sup.235 and U.sup.238, so that the peripheral speed of a rotor or cylinder for a separator must be greatly increased. This requirement dictates the use of material light in weight and high in strength. Maraging steel is typical of such materials.
More specifically, the peripheral speed of the rotor must be above 400 m/sec, such that the dimensional accuracy of the rotor must be strictly controlled or subjected to severe limitations in dimensions. Of course, dynamic and chemical characteristics and the cost of starting material and products must also be taken into account.
The characteristics of a rotor of the requisite type may be summarized as follows:
I. DIMENSIONAL ACCURACY,
II. FREEDOM OF RESIDUAL STRESS IN PRODUCTS,
III. DESIRED STRENGTH, INCLUDING STRENGTH AGAINST RUPTURE DUE TO INTERNAL PRESSURE, TENSILE STRENGTH, OR THE LIKE,
IV. LOW PRODUCTION COST AND EFFICIENT OR HIGH PRODUCTIVITY.
Heretofore, processes for producing maraging-steel rotors or cylinders for use in a uranium enriching centrifugal separator have included extrusion, welding or spinning. However, those prior art processes pose disadvantages which fail to meet the above requirements, particularly in accuracy of dimensions, efficiency of production and residual stress.
The drawing process is superior to other processes such as welding, spinning and extrusion for producing a cylindrical body. However, drawing has not been utilized because the resultant residual stress problem cannot be solved. In addition, the standard heat treatment of the 18% Ni-maraging-steel uses a solution heat treatment at 820.degree. C and a subsequent aging treatment at 490.degree. to 510.degree. C so that the elongation from such a solution heat treatment is considerably low, resulting in many defects, such as cracking and the like, when the steel is drawn. A temperature range other than that of the solution heat treatment which will produce improved elongation, will result in lowered strength of the final products, when the products are subjected to the final aging treatment step.
For these reasons, there have been many difficulties in the production of rotors or cylinders for a uranium enriching centrifugal separator, thus resulting in the use of time consuming, low efficiency processes and poor quality products.
A need exists therefore for a process for producing a maraging steel cylinder or rotor which overcomes the prior art difficulties.